"Two fighters from Islamic State detonated explosive belts at a gathering of Shi'ites in the city of Kabul in Afghanistan," said a brief statement on the group's Amaq news agency.
The attack was the worst in months and if confirmed as the work of Islamic State, would represent a major escalation for a group which has hitherto been largely confined to the eastern province of Nangarhar.
The explicit reference to the Hazara's Shia religious affiliation also represents a menacing departure for Afghanistan, where the bloody rivalry between Sunni and Shia typical of Iraq has been relatively rare, despite decades of war.
The Persian-speaking Hazara, estimated to make up about 9 percent of the population, are Afghanistan's third-largest minority but they have long suffered discrimination and thousands were killed under Taliban rule.
The Taliban, a fierce enemy of Islamic State, had issued a statement denying any involvement. "We would never take part in any incident that divides the Afghan people," Taliban spokesman Zabihullah Mujahid said.
The attack succeeded despite tight security which saw much of the city center sealed off with stacks of shipping containers and other obstacles and helicopters patrolling overhead.
It was the deadliest bombing seen in Kabul since April, when more than 60 people were killed in an attack on offices used by the security services. That was considered the worst single incident of its kind in Kabul since 2011.
Mohammad Ismail Kawousi, a spokesman for the ministry of public health, said at least 61 dead and 207 wounded had been taken to nearby hospitals.
